1. Field of the Invention
This invention relates to modulators particularly adapted to the modulation and demodulation of signals employed in carrier telephone systems, and more particularly to modulators (and demodulators) which provide a double sideband suppressed carrier output from unbalanced carrier and modulation frequency sources.
2. Description of the Prior Art
Typical of the prior-art modulators, which may be employed to obtain a DSBSC output signal, are those which are called ring modulators. In the early ring modulator circuits, rectifying devices, such as crystal diodes, were connected together in a bridge or a lattice arrangement and were positioned in the transmission path. Carrier suppression depended upon the careful matching of the diodes. This was expensive and not too satisfactory because the variations of the diode characteristics with temperature were not uniform as among the discrete elements. In addition, such an arrangement caused a transmission loss. Further, balanced transformers were required at the input and output of these modulators which added to their expense. Among other disadvantages of the ring type modulators, the problem of loss in the modulator was overcome by the invention described in U.S. Pat. No. 3,027,522, entitled Double Balanced Transistor Modulator, Boxall et al, Mar. 27, 1962. With reference to the circuit arrangements disclosed in the subject patent, it is important to note that transistors were used to replace the diode bridge, but that transformers were still necessary to provide the desired balance for suppression of the carrier, and, therefore, to obtain one of the required results.
More recently, a transistor modulator was disclosed which did not require the use of expensive bandpass suppression filters or balanced active and/or inactive devices. This modulator is disclosed in U.S. Pat. No. 3,719,903, entitled Double Sideband Modem With Either Suppressed Or Transmitted Carrier, Goodson, Mar. 6, 1973. This circuit employs a shunt switch modulator and it is well known that such a shunt switch modulation arrangement will not provide adequate suppression at high frequencies, such as, for example, 8 MHz, because the recombination time in the transistor does not allow the transistor to be shut off rapidly enough. Further, the level of transmitted carrier in the output signal depends upon the presence or absence of a DC bias in the modulator, as is taught by the invention.
Another modulator is the balanced modulator-demodulator circuit shown in FIGS. 5 and 6, page 7-392, of the "Linear Integrated Circuits Data Book", Motorola, Inc., 1972. This circuit does not provide adequate adjustment in the modulation frequency input so that the gain is independent of the amplitude of the modulation frequency signal. Thus, linearity suffers and the carrier suppression as well as the suppression of miscellaneous modulation products is not adequate for circuits which are to be employed in telecommunication systems.